'If a tree falls in the forest and there is no one there to hear it, does it make a sound?' ... stills from Gersht's video The Forest. Images courtesy of The Photographer's Gallery

A tree falls in the forest and the camera follows it down. The trunk crashes through the undergrowth in a cascade of leaves and dust, which hangs like smoke in a shaft of new sunlight. For a moment the birdsong and the dry background sizzle of insects fall silent. The last leaves twirl slowly earthward. Crash. There goes another one, this time noiselessly, and then another, with a cracking, rumbling bass note as it thuds into the earth. The trees fall this way and that. Slowly and inexorably toppling, they take longer to drop than one might imagine.

The camera keeps moving, tracking and panning between the trunks, some so close that they loom unfocused, like bodies passing before the camera. A moving shadow preceding a falling trunk as it drops into the frame. The camera's slow passage allows us to linger even on a fly, hovering and zigzagging in the air, the bright patches of green beech leaves, the sky seen through a break in the canopy. The editing is restrained, the track of the camera measured and purposeful.

But what is the purpose of Ori Gersht's The Forest, currently at London's Photographers' Gallery, and shown in conjunction with a series of photographs of landscapes, farm buildings and trees - which, by contrast, seem either snatched, juddery, whited-out, or whose images are swept into a sometimes almost indecipherable blur?

Gersht was born in Tel Aviv and has lived London for 17 years, arriving soon after completing his military service in Israel. He has photographed the pockmarked and war-scarred tower blocks of Sarajevo and the view of the sky from his own highrise flat in London; he has recorded sports stadiums in Germany and London, the flat-roofed, modernist schools built in Britain during the 1950s and 60s. He has also photographed the view from the windows of a passenger train on the route between Krakow and Auschwitz - trying, and often failing, to capture fleeting images as the train rushed through the Polish countryside.

Last winter, Gersht travelled to the small town of Kosov, among the vestiges of the great forest that once covered Europe, in western Ukraine. This is where Gersht's father-in-law, Gideon Engler, came from, and where, with his brother and father, he hid first in a hole in the ground and then in an attic, after Kosov was declared Judenrein, or cleansed of Jews, in November 1942. Soon after this Gideon's mother went missing, either shipped out to a death camp or led into the woods and shot. In the first Nazi aktion, 2,180 Jews from Kosov were taken to the mountain overlooking the town, in the forest of Moskalovka, ordered to undress at the "end of a whip", then to jump into one of two large pits where, one by one, they were shot. In order to save bullets, children were thrown in alive. The massacre continued the next day, in front of perhaps thousands of local spectators. The local schools were closed specially so the town's children could watch.

During the war, Baruch Engler, Gideon's father and a prominent Zionist activist, kept a diary, which Gersht read before his journey. Baruch was also a member of a "Help Committee", which, as a point of contact with the German and collaborationist Ukranian authorities, tried by one means or another to lessen the suffering of the Jewish populace. Engler's account of his years in hiding tells that he somehow got hold of some lengths of rope, so he and his two sons could hang themselves if capture seemed imminent. Gideon recalls his anxiety that, as a six-year-old, he was too small to hang himself. This terrible story is recounted in Jeremy Millar's long and thoughtful catalogue essay to Gersht's exhibition.

The photographs accompanying Gersht's film come from a series called Liquidation. What is being liquidated? Time, memory, people (no one is visible, only the signs of their habitation), the images, or reality itself? As the images smear into a blur, or judder, or approach blankness, it is hard to know whether the cause of these aberrations was the hand that held the camera shaking uncontrollably, or the view itself is being convulsed in some kind of seizure or earthquake.

As in Gerhard Richter's famous October 18 1977 cycle of blurred and frequently unfocused paintings, derived from photographs, it is as though the images were afflicted by a kind of hysterical blindness, an inability to look made manifest, palpable and concrete. The sensation is very like fainting. Something, at least, is making it difficult to focus or see clearly, both in Richter's paintings and Gersht's photographs.

My difficulty with Gersht's photographs is less to do with their metaphoric status, or what they represent, than with the artistic familiarity of his pictorial gambits. Too many artists have followed Richter's procedure (Paul Winstanley's blurry paintings of rooms, and Thomas Ruff's out-of-focus photographs, reshot from porn images, are but two examples). Richter himself has found it impossible to deal with the Holocaust, except obliquely. He never repainted the photos of concentration camps and piles of bodies that he collected in his atlas of photographic material. Instead, Richter focused on the German landscape, hillsides and forests, which recall both German Romantic painting and an idea of Heimat (homeland), which he then proceeded to paint in a distinctly unheimlich or uncanny way, as if the places themselves were somehow infected or undergoing obliteration.

Dragging up Richter here may seem unfair, but it is difficult to avoid his example, whatever great weight of subject matter is behind, or embedded in Gersht's photographs of the landscapes of Ukraine, or his earlier photographs, called White Noise, taken in Poland. Without their subtext, they lose their specificity. But this can be taken as another aspect of their meaning - in that their very emptiness is a record of absence, of the hidden, the disappearance of history. The eye passes over the photograph but cannot penetrate it. There is no mental adjustment we can make that will give it clarity, except by recourse to place, circumstance and the photographer's intention. And once we know the nature of these images, and what once happened in the forest, there's no turning back.

Both Millar and Steven Bode, who wrote a second essay in Gersht's catalogue, invoke the conundrum "If a tree falls in a forest and there is no one there to hear it, does it make a sound?" - a question often attributed to the 18th-century philosopher Bishop Berkeley. Berkeley, a churchman, wrote the dictum "esse est percipi": to be is to be perceived. The Ukranians and Germans circling the pits on the mountain certainly witnessed the Jews being herded to their deaths. Via Gersht, we witness the trees falling in the forest. But how far can you trust a pixillated, digital DVD - or a photograph come to that?

In Berkeley's universe, whether or not anyone hears the tree fall, God witnesses it. He might as well have said "stuff happens". What is at stake here is a notion of the real. Millar quotes Hitler's rhetorical remark, "Who remembers the extermination of the Armenians?" and notes that the Turkish novelist Orhan Pamuk remembers. It was Pamuk's mild statement of fact, and his question as to why no one wants to talk about it, that led him into the farcical, obscene brawl that terminated events in a Turkish courtroom last Friday. Call it what you will - extermination, genocide, ethnic cleansing - the false consciousness by which such events are at the same moment both remembered and suppressed is a denial of the real. Suppression is not the same thing as forgetting, though the total erasure of memory is the ultimate goal of the urge to suppress. As Harold Pinter said, in his recent Nobel address, of America's recent history of foreign adventures: "Nothing ever happened. Even while it was happening it wasn't happening. It didn't matter. It was of no interest." Somewhere, a tree is always falling

· Ori Gersht: The Clearing is at the Photographer's Gallery, London WC2, until February 5. Details: 020-7 831 1772 or photonet.org.uk.